The invention relates to an electomagnetic relay, including an armature being of the type that is, for example, adapted to adhere in the non-excited state to permanently magnetized poleshoes and contact springs actuated by the armature.
Magnetically polarized relays of this type are known. In such a case one or more permanent magnets are introduced into the magnetic circuit or circuits. The permanent magnet generates a flux in each of two magnetically conducting paths, which can be completed via an armature. In order to be able to move the armature and the contacts actuated thereby from one position to the other, an excitation field generated by a relay coil is superimposed on the field due to the permanent magnet. The advantage of such a relay resides in the fact that after switching over the contacts remain in their switched position owing to the adhesive forces due to the permanent magnet or magnets, without any need for further external exitation of the relay coil.
In order to ensure trouble-free functioning of such relays, care must be taken to ensure that on the one hand the sum of the magnet forces -- i.e. the forces exerted by the permanent magnet or magnets on the armature -- and the spring forces at any position of the armature always works in the direction of the poleshoes nearest the armature. This total force must be particularly large, especially at the two end positions of the armature, otherwise there is no guarantee that the armature would adhere properly in its position of rest. Although at some distance from the poleshoe, the total force decreases, it should not change its sign; otherwise, when lifting away only slightly from its end position and subjected to a mechanical shock, the armature could not be relied on to return to its end position and the switching position would change, a state of affairs which should not be brought about by mere mechanical shock. On the other hand, care must be taken to ensure that the sum of the excitation force -- i.e. the force resulting from the excitation and the permanent premagnetization and acting on the armature depending on the position thereof -- and the spring force works in such a direction during the whole of the stroke of the armature that it continues to the other end position. Only in such case do, in fact, forces obtain an excitation over the whole stroke of the armature, causing it consistently to move in the same direction. These conditions in respect of the permanent magnet force, the spring force and the excitation force are met when the curve of the spring force exerted on the axle determining the path of the armature lies between the curve of the permanent magnet force and the curve of the excitation force, without any of these curves intersecting.
With conventional relays, attempts are made in the interest of higher sensitivity to bring the curves of permanent magnet force and excitation force as close as possible to one another. These latter curves represent magnetization force vs. armature displacement. As however the (reflected) curve of the spring force must lie between them, it must be very accurately adjusted to the shape of the curves of the two magnetic forces. The spring characteristics must not intersect the magnetization vs. displacement characteristics as such intersection would mean a reversal of forces acting on the armature. As the magnetization curves are normally sharply curved, attempts have been made to effect this adjustment by the use of progressive springs which are difficult to manufacture. More often the rapid increase of the magnetic forces, as the end stop position is approached, has simply been limited by giving the magnet system the highest possible internal resistance.
A high internal magnetic resistance is achieved in the first place by the use of a permanent magnet of considerable effective length, thus making it needlessly bulky and expensive, and/or by operating the soft iron magnetic circuit at a high magnetic saturation, and/or by introducing into the magnetic circuit an additional air gap apart from the actual working air gap. By these means the shapes of the curves of permanent magnet force and excitation force are made flatter, so that one can make do with simple springs having linear characteristics.
A serious drawback of such high internal magnetic resistance resides in the tight spacing between the curve of the spring force from the curve of permanent magnetic force on the one hand and from the curve of the excitation force on the other hand, the latter representing the total effective force in the circumstances of electromagnetic energization of the relay. In the non-excited condition, the relay has little holding power when the armature is in the end stop position and is available for small power returning the armature to that position, if it has been lifted off e.g. on account of vibration. When the relay is excited, a small quantity only of energy is imparted to the armature, so that not only is the switching time prolonged, but most important of all, the speed with which the contacts open is slowed down, which increases the degree of burning of the contacts and consequently leads to a shortening of the useful life of the relay.
The feature which has the most decisive disadvantageous effect is the fact that the excitation flux also has to pass through the relatively elongated permanent magnet having a soft iron magnetic circuit operated at high saturation and/or through the additional air gaps, which requires a disproportionately greater magnetic flux to overcome these magnetic resistances and nullifies the gain in sensitivity aimed at and, consequently, results in a comparatively insensitive relay.
Relays constructed under the above-described principles thus result in constructions which, despite considerable attention devoted to adjustment, are sensitive to shock and have a relatively low switching speed, because the magneto-motive forces in the working air gaps were, in fact, kept at a very low level; nevertheless, the relays are comparatively insensitive, because a very much greater proportion of the excitation magnetic flux is uselessly dissipated in the magnetic circuit. The drawbacks of the known methods are, however, much more far reaching, as has been disclosed in numerous publications concerning efforts to remove these drawbacks.
The risk of intersection of relevant characteristics becomes greater the more attempts are made to render the relay more sensitive in this way, i.e. by bringing the curves of permanent magnetic and excitation force closer together. If, in fact, the curve of permanent magnetic force is shifted to lower levels owing to leakage of the magnetic properties or the like, the curves intersect immediately. This has lead to numerous efforts to compensate the magnets by temperature compensation etc. which is a very arduous procedure. Intersection of curves similarly occurs between the mirror image of the curve of the spring force and the curve of the electromagnetic excitation force, if during the operating period even only moderate burning of the contacts takes place. During burning of the contacts, the points of contact making and breaking actually shift, i.e. the points en route to which the spring forces are decreasing to zero so that during this time the springs are subject to decreasing tension. Consequently, the spacing between the mirrored spring force curve and the permanent magnet curve and consequently latterly the adhesive force becomes larger, while the spacing from the excitation force is exceeded.
The shifting of the curve or characteristics of spring pressure due to burning at the contacts is highly undesirable, since any permanent adjustment is out of the question. Moreover, when using a progressive spring system, the characteristic of which is continually changing, it is never known exactly where the intersection will arise. If it is situated in the vicinity of the armature and stop positions, the relay does not switch at all. If it lies somewhere between the end stops, then the relay is uncertain in operation. A mere shock or friction may cause it to fail and the armature will come to rest in an unwanted switching position. This faulty operation happens usually where the contacts remain closed under the smallest contact pressure.
If the armature does not come to rest, it will move in a sporadically, creeping fashion. If the contacts are operating under a high loading such creeping has a particularly disastrous effect on their condition. In order to avoid intersection of spring and magnetization characteristics resort is had to higher excitation capacities, but then the operating voltage must be readjusted from time to time by the user. This is a very unrealistic requirement. Consequently, when attempting to work at the specified and advertised response sensitivity, suitable additional precautions have to be taken at the outset, so that the lower excitation loading which is usually bought at considerable expense cannot be made use of at all.
It follows from the foregoing that the adhesive force cannot even be stated with any degree of reproducible accuracy with such relays. The response sensitivity can be defined with some sort of accuracy only, because it depends only in part on the magnetomotive driving power and is determined to a predominating extent by the resistance of the magnetic circuit. The result of this is that when operating at levels above or below the rated excitation, it is quite impossible to predict how the relay will behave, because, -- especially when manufacturing tolerances, saturation phenomena, the temperature dependency of the material from which the magnetic circuit consists come into play -- an indefinite fraction only of the excitation power can be made effective for the generation of magnetomotive force. The adhesive force and relay behaviour are the less defined with regard to changes in the response excitation, the greater are the efforts made to adjust the shape of the curve of spring power to the shape of the curve of permanent magnet force, in order to make the relay sensitive. Fluctuations in the permanent and/or spring power of a few percent give rise to considerable variations in the adhesive force and also of the effective excitation required, owing to the effect of the disparity.
However, all such relays which have been made sensitive by causing the curve of spring force to conform closely to the curves of permanent magnet force and the excitation force have the fundamental drawback that over long stretches of the stroke of the armature the difference between the spring force and the excitation force is small, causing the force/stroke-integral, which defines the kinetic energy transmitted to the armature, to be small. This once again means relatively slow switching times and low speeds of contact separation.
For reasons of symmetry, relays with permanent magnetization and particularly for pulse operation and depending on direction of energization upon the desired switching state to be attained, are constructed with a swivel or pivotal armature, wherein each end of the armature abuts to poleshoe structure in each of the two switching states and positions; that is to say, such abutment is supposed to occur; otherwise the adhesive force will differ from the desired condition.
Journalling of the pivotal armature is absolutely necessary in numerous applications, invariably for example when importance is attached to signal sequence-controlled contact. On the other hand, however, owing to the journalling of the pivotal armature, the problem arises that when it is in contact with two of its abutting surfaces, the position of the armature is invariably over-defined or controlled from the static point of view. This is because in such a position the armature is not only supported at its pivotal axis, but also by the abutting surfaces, resulting, therefore, in a three-point support.
It may now happen that the rotational axis of a pivotal armature so mounted in a relay is not in absolutely accurate alignment with the abutting surfaces; the armature does not, therefore, come into perfect contact with the abutting surfaces so leaving undesirable gair gaps. Manufacturing and assembly tolerances must inevitably be taken into account during the manufacture of such relays and as a rule there is no guarantee that the rotational axle of the pivotal armature will, in fact, be accurately journalled in its bearings. On the other hand, however, it is usually very difficult to correct the disposition of the rotational axle, particularly when the pivotal armature is mounted in an aperture made in the carrier carrying the relay coil.
It can readily be seen that uncertainty in the engagement between both ends of a swivel armature and the poleshoes, compounds the problems regarding magnetic attraction vs. displacement characteristics as outlined earlier.